I did this when I bought my first Mac, an e Mac about 10 years ago. You need a separate cassette deck, like you would find in an old hi fi system. It MUST have output on the rear to send the audio to your Mac. I used a cable that was two phonos ( for the rear of the tape deck ) which are red and white going in to a 3.5mm jack, like a headphone plug. Connect that to your line in on your Mac ( not your headphone port) as the line in takes audio in, the headphone jack doesn't. Then I used Peak to record the tapes and edit. I know that Garage band will also work, I messed around with it for a while but I'm more used to Peak. I know Peak has full editing options and I would guess Garage Band would probs have less options but you should be ok, if not you might need to buy some audio editing software like Peak ( not cheap ) .
Takes a bit of experimenting with to set up, that's the only difficult bit but once you're up and running, its very straight forward.